This chapter reviews in vitro and in vivo differences in tibiofemoral kinematics between a low-conforming total knee arthroplasty (TKA) implanted with calipered kinematic alignment (KA) and the native knee. Kinematics are defined by laxities and resting positions of the tibia with respect to the femur, and the anterior-posterior (A-P) movements of the femoral condyles with respect to the tibia. Laxities are the translations and rotations that are measured under forces and moments applied to the tibia, respectively. The “resting position” is the position and orientation of the tibia relative to the femur without external load other than compression. The A-P movement of a femoral condyle is indicated by the change in position of the lowest point (actually closest point) of a femoral condyle with respect to the plane of the tibial baseplate. The low-conforming TKA is representative of the most common designs in use. The features are an insert with shallow medial and lateral concavities fixed to an asymmetrically shaped tibial baseplate, and a femoral component that retains the posterior cruciate ligament (PCL). The in vitro study showed that the low-conforming TKA exhibited greater anterior tibial translation and internal and external rotation at 30-degrees flexion and different resting positions than those of the native knee. The in vivo fluoroscopic study showed loading of the posterolateral insert rim by the prosthetic femoral condyle in 16% of TKAs, which was partly caused by undercoverage of the posterolateral tibia by the baseplate and insert. A strategy for restoring anterior laxity and resting positions after excising the anterior cruciate ligament (ACL) is to use a medial ball-in-socket femoral component coupled with a 1:1 conforming medial insert concavity. A strategy for reducing the risk of posterolateral rim loading is to use an asymmetric baseplate that covers the posterolateral tibial resection, and an insert with a flat lateral surface without a rim that fully covers the baseplate.
In Vitro Differences in Laxities and Resting Positions Between a Low-Conforming Total Knee Arthroplasty Implanted With Calipered Kinematic Alignment and the Native Knee Analyzed in the Same Cadaveric Knee
describes and shows the methods used to measure tibiofemoral laxities and resting positions.
A custom load application system measured eight laxities and four resting positions at five flexion angles in thirteen cadaveric knees during passive motion from full extension to 120-degrees flexion. The eight laxities consisted of varus-valgus (V-V) and internal-external (I-E) rotations and A-P and compression-distraction (C-D) translations measured under prescribed loads of 5 Nm for V-V, 3 Nm for I-E, 45 N for A-P, and 100 N for C-D. A nominal 120-N compression force was applied by loading tendons to stabilize the knee during testing. Laxities and resting positions were measured first on the native knee and then after implantation of the low-conforming TKA using calipered KA, which enabled paired analyses between the two knee conditions within each specimen.
The low-conforming TKA implanted with calipered KA restored laxities comparable to those of the native knee with the exception of anterior translation and I-E rotation ( Fig. 9.1 ). At 30 degrees flexion, the low-conforming TKA exhibited 2-mm more anterior laxity ( Fig. 9.1A ), 2 degrees more tibial internal rotation ( Fig. 9.1C ), and 4 degrees more external rotation ( Fig. 9.1D ) than those of the native knee.
The low-conforming TKA restored comparable tibial resting positions to those of the native knee with the exception of anterior translation and I-E rotation ( Fig. 9.2 ). The position of the tibia of the low-conforming TKA was 4 mm more anterior at 0-degrees flexion and 2 to 3 mm more posterior at 60-degrees (or more) flexion than the native knee ( Fig. 9.2A ). The rotation of the tibia of the low-conforming TKA was 4 degrees more internally rotated at 0-degrees flexion and 5 degrees more externally rotated at 30-degrees flexion than the native knee ( Fig. 9.2B ).